When talking with students, or people who just love light but don’t know much about it, there are a few terms that are used sometimes incorrectly – “That table has a high illuminance value,” or “you can measure the luminance coming from that light source.” Not quite right. Do you know how to use these integral lighting terms?

It’s easiest to remember if you think of it in terms of what is exactly happening when a beam of light strikes a surface. Let’s look at this small image:

In this example, we are looking at one beam of light hitting a surface. The beam of light is red, and the surface, obviously is green. The red arrow represents the incident light, or the beam(s) of light striking the surface. Another time I’ll write about the Law of Reflection and what it means, but also look at this diagram of what happens when a beam of light is reflected:

Figuring out the difference between luminance and IL-luminance is as simple as remembering what has happened to the beam of light; the incident of light hitting a surface – a beam of light that is travelling towards a surface – is what is measured when talking about illuminance. Illuminance is measured as the amount of light striking a surface. The incident light. If we had a light meter and wanted to measure the amount of light that was striking the surface we were measuring, we would hold the little white bubble towards the device shooting the light. These meters are also called “incident meters” for a reason.

On the other side of this, luminance, is what we measure off of the surface that has light hitting it. Luminance is the measurement of the product of the incident light and the surface – anything that is reflected. Luminance is also considered the human perception of brightness, or how bright we perceive the light that is reflected off of the surface.

Make sense? Think of it like this – IL-Luminance, IL, I = Incident Light. Illuminance is measuring the incident light. Luminance is what’s leaving the surface – L = leaving. Illuminance is measuring incident, luminance is measuring what’s leaving.

Here’s the loopy metaphorical note I made to myself as I read your excellent explanation — is this essentially anywhere near correct?

The “incident” is the arrival of light, as in: “Hello! I’m a light beam! Here I am! Bam!” “Illuminance” is the customs agent asking to see the beam’s passport, stamping it, and asking how much currency it’s bringing in. “Luminance” is how much of that currency (light) the beam then gives to its family that’s waiting at the other side of the room ?

She’s actually quite intelligent. Making a metaphor to remember concepts ensures that you remember the information for a longer period of time. Here’s something that you should remember: the only reason someone would insult a stranger is because of their own insecurity.

So what is wrong with saying “you can measure the luminance coming from that light source.” – reading the article it seems like the correct use of the term – we’re asking how much light is leaving the light source?